Weight paint not working (bug?)
When you're weight painting, you're painting into Vertex Groups.

The vertices you can't paint don't belong to  a group.

There's a difference between a vertex belonging to a group with 0 weight, and a vertex not belonging to a group at all.

If a vertex is not assigned to the active group, you can't paint into it.

Assign vertices (with a given weight) to a group in the Data tab > 'Vertex Groups' panel.

It's very confusing, because by default, unassigned vertices are colored blue, like 0-weight vertices.

You can fix that.. in the Overlays dropdown there's an option, even more confusingly called 'Zero Weights'. (IMO, it should be called 'Unassigned Vertices'.) Anyway, you can use it to display vertices unassigned to the active group, or those unassigned to any group, in black, not blue.